Solid-State PULSE COUNTING FM RADIO 1966~2016

Armstrong Micro FM

The Sinclair Micro FM (c1966) has been "reborn" in 2016 and is available as a constructors kit at:
Armstrong Radio    US$18 + US$6 postage.  ( sometimes also listed on Ebay)

Circuit re-draft

Some history: Sinclair Micro FM. (c1966)

Re-draft of an original schematic:

Sourced from: Cool 386 webpage
There was a commercially made kit, the Sinclair Micro FM, for sale in the UK during the late 1960's.
It would be familiar to anyone reading 1960's English electronics magazines.
It was a pulse counting circuit with untuned RF amp, autodyne converter,
and used Micro Alloy Transistors.
Claimed sensitivity was 3uV and the receiver could drive an earphone and had a line out socket for an external amplifier.
The few reports I've found on the internet suggest sensitivity was less than this.
The PP5 battery required is obsolete, and was apparently already difficult to obtain
at the time the Micro FM was being sold.
For those wishing to try building it, L1 and L2 are 14 turns around a 470K resistor.
Wire gauge is not specified, but I'd guess about 25B&S.
L3 is six turns around a plastic former with adjustable ferrite slug.
The former and slug would appear to be similar to the modern Neosid type of thing.
Brief operation is as follows; Tr1 is an untuned RF amplifier to prevent aerial loading,
and to provide some gain (I'd guess about 10dB max).
Tr2 is a self oscillating converter (i.e. autodyne).
RF is filtered off by C10, leaving a 120 kHz IF.
This is amplified by Tr3 and Tr4.
Tr5 also provides amplification but is the limiting stage
to provide a clipped waveform to the pulse counting detector.
This consists of C17 as the differentiator, and D1 and D2 which are the diode pump.
Tr6 provides audio amplification, with C20 integrating the detected pulses into a smooth audio waveform.
Line level audio is available from this stage.
Tr7 is merely an earphone driving stage.
AFC is provided by using the DC at Tr6's collector to control Tr2's oscillation frequency.
R7 and C4 are low pass filter to remove the audio component from the control voltage.

Link to: Cool386's excellent (as always) analysis of the Armstrong pulse-count receiver

Link to: Valve based pulse-counting receiver design

From the same era (c1966): build a TV receiver from a kit!